1. Field of the Invention
The present invention is related to telecommunications; more specifically, communications involving DTMF dual-tone multiple frequency signals.
2. Description of the Related Art
DTMF signaling is commonly used in the telephone system. Each DTMF digit or signal contains a pair of tones, which are selected from a low group of four frequencies (697 Hz, 770 Hz, and 941 Hz) and a high group of four frequencies (1209 Hz, 1336 Hz, and 1633 Hz), respectively. DTMF signaling is used for call setup and control, sometimes in the presence of speech and noise. In many applications it is necessary to detect or remove DTMF signals.
Currently, some techniques exist for detecting DTMF input signals within an input audio signal using Goertzel algorithms, tuned filters, and adaptive filters. The Goertzel algorithm is widely used for DTUF signal detection with commercial DSP processors "Add DTMF generation and decoding to DSP-.mu.P designs", P. Mock, EDN, Mar. 21, 1985 and "Dual-Tone Multifrequency Receiver Using the DSP16 Digital Signal Processor", G. Smith, AT&T Applicaton Note, 1989. It is a modified Fourier transform, which computes the content of a specific frequency, instead of the whole spectrum. The energy in that frequency is used to decide its presence through a threshold. A more advanced technique for DTNF signal detection is described in U.S. Pat. 5,392,248 (S. Park, and D. Funderburk, "DTMF Detection Having Sample Rate Decimation and Adaptive Tone Detection," issued Feb. 1995, filed Nov. 25, 1991.) It separates the incoming signal into two bands with a filter, then adaptively determines the frequency and the magnitude of any existing tone. However, all of these techniques solely detect DTMF signals and do not remove the DTMF signals or mute the speech from the input audio signal within the same operation which is useful for many voice communications systems for quickening and optimizing call processing and other operations. Additionally, many of these solutions require several DSP iterations and components to work effectively which increases costs and reduces speed. Therefore, there is a need to effectively integrate DTMF signal detection and removal within a single operation to provide the desired voice communications applications without unduly comprising speed, accuracy, or cost.